The present invention relates to a method for continuously dispersing fine-particle-sized solids in a liquid, and, in particular, a liquid component of a two-component reactive plastic, such as for example a polycondensation or polyaddition system, and more particularly in the polyol component of a polyurethane system.
In two-component plastic, the two plastic components which are reactive with one another are usually mixed in liquid form in a mixhead in specific quantitative ratios and then reacted, optionally after having been introduced into a mold. Catalysts or heat are optionally used to accelerate the reactions. In the case of polyurethane plastics, such as for example polyurethane foams, one of the components consists of an isocyanate or an isocyanate prepolymer and the other component of a polyol, the two components being metered into the mixhead in a specific ratio.
Inorganic, e.g. mineral fillers, or organic fillers are frequently added to such two-component plastics for the purpose of modifying their property profile. U.S. Pat. No. 5,152,943 (corresponding to German Offenlegungschrift 3,841,671) describes mixing the filler with one of the reactive components of the two-component system in a preliminary mixer and feeding the filler-containing component into the mixhead. The filler is fed into the preliminary mixer by means of a screw in which the filler is compacted, the metering of the filler being effected by controlling the speed of rotation of the screw as a function of the pressure in the preliminary mixer.
European Patent 431,388 describes preventing any undesirable introduction of air into the preliminary mixer in the process according to the '943 patent by designing the jacket of the compacting screw at the outlet end upstream of the preliminary mixer in the form of a porous microfilter consisting of a porous sintered metal or a plastic and surrounding it with an annular suction channel for removing the air from the compacted filler by suction. The disadvantage of this process is that when fine particle-sized fillers are used or fillers with a high content of very fine particles, the microfilter becomes blocked within relatively short periods of time and must be replaced.
The object of the present invention was therefore to prevent the above-mentioned disadvantages. More particularly, the object was to prevent the undesirable air present in the intermediate spaces between the solid particles from entering the preliminary mixer, while maintaining the metering accuracy of the metering screw for the fine particle-sized solids (fillers). This object is achieved by the present invention.